The role of beta-receptor and calcium-entry-blocking agents in acute myocardial infarction in the thrombolytic era: can the results of thrombolytic reperfusion be enhanced?

Mechanical and electrical complications of acute myocardial infarction

Although much of the current enthusiasm in the management of acute myocardial infarction is related to revascularization strategies, mechanical and electrical complications continue to pose a major threat to recovery in some patients. Some of the major complications of acute myocardial infarction are cardiogenic shock, rupture of the free wall and pseudoaneurysm, rupture of the ventricular septum, acute mitral regurgitation, right ventricular myocardial infarction, infarct expansion or extension, pericarditis and tamponade, peri-infarction hypertension, and tachyarrhythmias and bradyarrhythmias. For each of these complications, general guidelines for diagnosis and management are offered. Early, aggressive, and judicious treatment of these complications may substantially decrease the morbidity and mortality associated with acute myocardial infarction.

Reperfusion in acute myocardial infarction

During the past decade, the general acceptance of the primary role of thrombosis in acute myocardial infarction (AMI) has led to intense interest in the potential efficacy of reperfusion therapy, particularly thrombolytic therapy, in AMI. Accumulating evidence indicates that systemic thrombolytic therapy administered early after the onset of symptoms of AMI can restore infarct-related artery patency, salvage myocardium, and reduce mortality. Recommendations about the proper use of thrombolytic therapy, contraindications, and concomitant therapies (such as aspirin, heparin, nitrates, beta-adrenergic blocking agents, and calcium channel blockers) are reviewed. Although percutaneous transluminal coronary angioplasty (PTCA) is useful for subsets of patients with AMI (for example, patients with anterior infarctions with persistent occlusion of the infarct-related artery after thrombolytic therapy and those with cardiogenic shock), a conservative strategy, including angiography and PTCA only for postinfarction ischemia, is indicated for most patients with AMI in whom initial thrombolytic therapy is apparently successful. The use of PTCA after failed thrombolysis or as direct therapy for AMI seems promising, although further comparisons of PTCA and intravenous thrombolytic therapy are needed. Ongoing studies should help further define the risk-to-benefit ratio of various reperfusion strategies in different subsets of patients, define time limitations for reperfusion therapy, and provide data on therapeutic modalities that will limit reperfusion injury and therefore enhance salvage of myocardium.

Acute myocardial infarction: initial manifestations, management, and prognosis

In patients with suspected acute myocardial infarction (AMI), obtaining a thorough history is important for identifying both the cause of chest pain and any concurrent conditions that may complicate the management. Physical examination--including cardiac auscultation and determining the status of the peripheral vasculature--is important as a guide to immediate management and as a baseline for future comparison. The differential diagnosis of AMI is extensive, and various laboratory tests, such as electrocardiography, cardiac enzymes, radionuclide techniques, echocardiography, and cardiac catheterization, can aid in the diagnosis. The routine management of patients with AMI can include medical therapy with antithrombotic agents, nitrates, beta-adrenergic blockers, or calcium channel blocking agents. The major differences between Q-wave and non-Q-wave infarction are discussed. Some factors that affect early and late prognosis in patients with AMI are age of the patient, residual left ventricular function, residual myocardial ischemia, and substrates for sustained ventricular arrhythmias. Although much of the current enthusiasm in management of AMI is related to revascularization strategies, other important aspects of diagnosis and treatment should not be overlooked.

Reperfusion injury and its pharmacologic modification

Reperfusion injury includes a spectrum of events, such as reperfusion arrhythmias, vascular damage and no-reflow, and myocardial functional stunning. The concept of reperfusion injury remains controversial with many proposed mechanisms when applied to humans, whereas in animal models, there are two main proposed mechanisms: calcium over-load and formation of oxygen free radicals. To prove that reperfusion injury is specifically caused by reperfusion would require evidence that an intervention given at the time of reperfusion can diminish or abolish the injury as in the case of arrhythmias, which are thought to be mediated by excess recycling of cytosolic calcium with delayed afterdepolarizations and ventricular automaticity. In the case of myocardial stunning, the phenomenon may be mediated, at least in part, by a burst of free radicals formed within the first minute of reperfusion and improved by free radical scavengers given at the time of reperfusion. The alternate hypothesis is that cytosolic calcium overload damages mechanisms for normal intracellular calcium regulation so that the stunned myocardium responds to agents that are thought to increase intracellular cytosolic calcium, such as beta-receptor agonists. A further component of reperfusion injury, under active investigation, is microvascular damage with alterations at the level of platelets, leukocytes, and endothelial integrity. From the therapeutic point of view, the divergent results of experimental interventions and the possibility that the abrupt onset of reperfusion in animals differs from the situation in humans with thrombolysis means that the best way currently available to limit reperfusion injury is by minimizing the ischemic period by early reperfusion and by optimizing the metabolic status of the ischemic myocardium at the end of the ischemic period.